Dosimetry

Dosimetry / Personal dosimetry

According to radiation protection rules all persons who may be exposed to nuclear radiation in a controlled area must register the radiation dose received during presence in the controlled area and limiting values of radiation dose must not be exceeded. Moreover, it is recommended that every person who may be exposed to enhanced radiation level even outside of controlled areas should  be equipped with a dosimeter. Dosimetric surveillance can be volunteering or requested by law. For requested dosimetry one must use registered and calibrated dosimeters which can be purchased as such. Sometimes these dosimeters can be bought from supervising agencies.

The whole-body dose is measured with a dosimeter that is attached to a representative area of the body. A dosimeter is an active or passive device serving the quantitative measurement of doserate and integration of the dose. In addition to whole-body dosimetry one may use specific dosimeters for particularly exposed organs, such as figerring dosimeter, eye dosimeter etc. Most personal dosimeters measure the doserate (i.e. the dose per unit time in [µSv/h]) and they calculate the dose by integration of the doserate from gamma radiation in the energy range ~45 keV to ~3 MeV. Limitations with respect to the energy range stem from the technical details of the detector employed; very few dosimeters register photons with energies below 30 keV.

An important criterion for users in qualifying dosimeters is the immediate availability of measured results (direct reading dosimeter). This is particularly important when dosimeters are used for short-term surveillance. Another criterion is the physical process which is employed for quantification of radiation doserate/dose.

Pen Dosimeter, Film Badge

Pen Dosimeters are directly reading mechanical meters in which an electrically charged “hair” of quartz glass bends in front of a counter electrode. Ionizing radiation will discharge the hair and it bends back. An optical system projects the location of the hair onto a scale for direct reading. The hair can be reset by re-appling charge. Depending on structural materials used in the pen dosimeter it can be used for measurement of gamma-rays or X-rays. There is a special version (Sievert chamber) that measures the beta particles dose.

Film Dosimeters are accumulating dosimeters for long-term supervision of professionally exposed workers. An X-ray film sitting in the support badge is blackened by radiation. From the amount of blackening one can read the radiation dose very reliably. Using various absorber materials (filters) in front of the film  one can distinguish low-energy gamma-ray radiation from higher energies and also beta radiation. The dynamic range of film dosimeters is small.  Most professionally exposed persons in Germany wear film badges which are evaluated and often also supplied by a central federal agency.

Thermoluminescence Dosimeters (TLD) are accumulating dosimeters in which electrons are promoted by incoming radiation to meta-stable excited states. The electrons are trapped in the meta-stable state until they are thermally activited and fall back to the ground state; the energy difference between meta-stable and ground state is emitted in the form of light. The intensity of light is a measure of the accumulated radiation dose in the TLD chip. To “read out” the TLD chip the chip is heated up and the amount of light is continuously measured as a function of temperature; the resulting curve showing light intensity vs. temperature is called the glow-curve. From details of the glow-curve and the total area below the curve one can determine the radiation dose very reliably.
In some of the currently used TLD materials the meta-stable states have limited stability; these materials can only be used for short-term integration times of several weeks or months.

A similar working principle like TLD is applied in Photoluminescence Dosimeters where the emission of light is stimulated by photo-activation.

For dosimetry of beta- and X-ray radiation one uses thermally stimulated Exo-Electron-Dosimetry (Tsee) where thin slices of BeO ceramics having special dopants are used as integrating dosimetry materials. Like in TLD material excited electrons are captured in meta-stable states. When the ceramic material is heated these electrons are released from the ceramic and measured with an open proportional counter. The number of released electrons is a measure of the accumulated dose. The detection limit of Tsee-dosimeters is around 10 µSv.

The radiochromic Fricke Dosimeter uses the oxidation of Fe2+ into Fe3+ ions in aqueous solution by ionising radiation for measurement of the deposited dose. The amount of Fe3+ produced is determined by absorption spectrometry for bands at 224 nm and 303 nm. The Fricke Dosimeter is sensitive for very high radiation doses ranging from 0.5 kGy to 400 kGy.

The formation of free radicals is employed in the Alanin Dosimeter for determination of the radiation dose. The density of radicals is a measure of the dose; it is determined with Electron-Paramagnetic-Resonance. The Alanin Dosimeter has linear response between 10 Gy and 50 Gy. It is particularly suitable for certain medical applications.

Electronic Dosimeters

Nowadays direct reading electronic dosimeters are most frequently employed. Various types of detectors are employed in different brands. Most electronic dosimeters use Geiger-Mueller tubes for measurement of the doserate, sometimes supported or replaced by NaI(Tl), BGO, CZT or other detectors. The neutron doserate can be measured with a 3He counter tube. Some electronic dosimeters automatically switch between a scintillation detector for low-doserate application and the GM-tube for high doserates.
Electronic Dosimeters indicate the current doserate and/or the accumuated dose digitally on the LCD-screen. The small battery operated units are permanently switched on. The integrated optical, acoustic and vibration warning function makes sure that excessive radiation levels are avoided.

Multi-Function Dosimeters

There are portable Multi-Function Dosimeters where features of powerful electronic dosimeters are enhanced by additional functions. Typical options are storage of large amounts of measured data points together with e.g. time stamp and GPS coordinates, or energy dispersive gamma-ray or alpha-particle spectra for on-line determination of nuclides. A Bluetooth or Network function may allow transfer of measured data from the dosimeter to a central supervisor (reach back function). Additional functions such as an activity search function help to find hidden sources with optical, acoustic and vibrational indication.

Radon Dosimetry

Personal dosimetry of radon and its progeny is a very special field in dosimetry. It is particularly important in underground mining applications, in publicly accessible visitor caves, is waterworks and other places accumulating higher radon concentrations.

Passive accumulating dosimeters are nuclear track detectors (mica, foils, films) where alpha particles from the decay of radon or progeny produce etchable and then visible tracks, or collecting cans filled with activated charcoal where radon gas is trapped and accumulated; gamma-ray spectra from its progeny are then measured after typically one week exposure time.

Active measurement of radon in air is made with pulsed ionisation chambers where air is used as the counting gas. As an alternative one can quantify radon via its progeny which is deposited as the aerosol-attached fraction on a filter paper. All active measurements of radon and progeny allow discrimination between  220Rn (232Th progeny) and 222Rn (238U progeny). Spectrometry of attached progeny has the advantage that one can quantify contributions to the dose of individual members of the decay chain. In the case of uranium decay this are contributions from 238U, 234U, 230Th, 226Ra, 222Rn, 218Po, 214Po and 210Po. One finds all nuclides on filters loaded with dust in uranium mining. Using modern possibilities (see ALPS) of spectrum analysis one can make such nuclide-specific quantification even without chemical treatment of the filter sample.

In waterworks one typically finds only polonium isotopes; if the water contains  220Rn then one finds also 216Po, 212Bi and 212Po.

Gadgets

There are gadget dosimeters for every day use such as a wrist-watch with integrated dosimeter or an additional hardware extension for the iPhone which converts the phone into a networking dosimeter.

A quaint but feasible dosimetry solution is an app for the smart phone that uses the photo-chip as detector for gamma-rays. As there are several conditions which apply for this application the precision of measurement is limited and smart phones cannot replace a personal dosimeter.